Lithographic apparatus, device manufacturing method and associated data processing apparatus and computer program product

ABSTRACT

A lithographic apparatus includes a number of sensors for measuring positions of features on a substrate prior to applying a pattern. Each sensor includes an imaging optical system. Position measurements are extracted from pixel data supplied by an image detector in each sensor. The imaging optical system includes one or more light field modulating elements and the processor processes the pixel data as a light-field image to extract the position measurements. The data processor may derive from each light-field image a focused image of a feature on the substrate, measuring positions of several features simultaneously, even though the substrate is not at the same level below all the sensors. The processor can also include corrections to reduce depth dependency of an apparent position of the feature include a viewpoint correction. The data processor can also derive measurements of heights of features on the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase entry of PCT patentapplication no. PCT/EP2015/062846, which was filed on Jun. 9, 2015,which claims the benefit of priority of European patent application no.14177232.7, which was filed on Jul. 16, 2014 and European patentapplication no. 14199539.9, which was filed on Dec. 22, 2014 and whichare incorporated herein in their entirety by reference.

BACKGROUND

Field of the Invention

The present invention relates to a lithographic apparatus. The presentinvention further relates to methods of manufacturing devices usinglithographic apparatus calibrated by such a method, and to dataprocessing apparatuses and computer program products for implementingparts of such a method.

Background Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,comprising part of, one, or several dies) on a substrate (e.g., asilicon wafer). Transfer of the pattern is typically via imaging onto alayer of radiation-sensitive material (resist) provided on thesubstrate. In general, a single substrate will contain a network ofadjacent target portions that are successively patterned. Knownlithographic apparatus include so-called steppers, in which each targetportion is irradiated by exposing an entire pattern onto the targetportion at one time, and so-called scanners, in which each targetportion is irradiated by scanning the pattern through a radiation beamin a given direction (the “scanning”-direction) while synchronouslyscanning the substrate parallel or anti-parallel to this direction.

A key requirement of the lithographic process is to be able to positiona pattern in the correct position relative to features formed inprevious layers on the substrate. Alignment sensors are provided forthis purpose. As device structures become ever smaller, alignmentspecifications become ever tighter. Similarly, at least in the case ofoptical lithography where the patterns are applied using an opticalprojection system, a key requirement is to measure accurately the localheight of the substrate, so that the pattern can be optically focusedonto the resist layer.

Known forms of alignment sensing arrangements are disclosed in publishedpatent applications such as US2008/043212A1 (Shibazaki) and US2011/013165A1 (Kaneko), both of which are incorporated herein byreference. To reduce the time taken for measuring many positions acrossthe substrate, these known examples provide multiple alignment sensors,operable in parallel. To obtain a highly accurate position measurement,each alignment sensor should be focused on the substrate surface (or ona target mark beneath the substrate surface). However, since thesubstrate is generally not perfectly flat, it is impossible for all ofthe alignment sensors to capture focused images of several marks at thesame time. In the published applications, the alignment sensors areoperated to capture multiple images of the same marks, each time with adifferent height (focus) setting. The best measurement of each mark isselected from the image where the corresponding sensor was in bestfocus. While the known system can provide accurate positionmeasurements, the time taken for the multiple measurements can cause areduction in throughput of substrates in the manufacturing process.

SUMMARY OF THE INVENTION

It is an object of the invention to enable the provision of an alignmentsensing arrangement that can make position measurements on a pluralityof marks in a single pass.

According to an aspect of the invention, there is provided alithographic apparatus for applying a pattern onto a substrate, theapparatus including:

at least one sensor for measuring positions of features on the substrateprior to applying said pattern, said sensor comprising an imagingoptical system and an image detector for capturing an image formed bysaid imaging optical system;

a data processor for extracting position measurements from pixel datasupplied by said image detector; and

a controller arranged to control the lithographic apparatus to applysaid pattern to the substrate using the positions measured by saidsensor,

wherein said imaging optical system includes one or more light fieldmodulating elements and said data processor is arranged to process saidpixel data as a light-field image to extract said position measurements.

In some embodiments the position measurements are used for alignment.The data processor may be arranged for example to derive from saidlight-field image a focused image of a feature on the substrate, and toindicate a position of the feature based on said focused image indirections transverse to an optical axis of said imaging optical system.The data processor may be arranged to include in said positionmeasurement a correction to reduce a depth dependency of an apparentposition of the feature in the detected image. The data processor may bearranged to derive from said light-field image an image of a feature onthe substrate with a viewpoint corrected.

In some embodiments the position measurement is used for focus control.For example said data processor may be arranged to derive from saidlight-field image a measurement of height of a feature on the substrate,the dimension of height being substantially parallel to an optical axisof the imaging optical system, the controller using said measurement ofheight to control focusing of a pattern applied by said lithographicapparatus.

According to an aspect of the invention, there is provided a devicemanufacturing method comprising applying patterns in successive layerson a substrate, and processing the substrate to produce functionaldevice features, wherein the step of applying a pattern in at least oneof said layers comprises:

-   (a) measuring positions of features on the substrate in a    lithographic apparatus, using at least one light-field imaging    sensor;-   (b) extracting position measurements from light-field image data    obtained using the sensor; and-   (c) controlling the lithographic apparatus to apply said pattern to    the substrate using the positions measured by said alignment sensor.

The invention further provides a data processing system comprising oneor more processors programmed to implement the data processor of alithographic apparatus according to the invention as set forth above.

The invention further provides a computer program product comprisingmachine-readable instructions for causing one or more processors toperform the step (b) of a method according to the invention as set forthabove.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 depicts a lithographic cell or cluster incorporating theapparatus of FIG. 1;

FIG. 3 illustrates schematically a measurement process and an exposureprocess in the apparatus of FIG. 1, according to known practice andmodified in accordance with an embodiment of the present invention;

FIG. 4(a) is a schematic plan view of a sensing arrangement in theapparatus of FIG. 1;

FIG. 4(b) is a schematic plan view of measurement and exposure stages inan alternative lithographic apparatus, usable with the sensingarrangement of FIG. 4(a) in another embodiment of the invention;

FIG. 5 is a schematic side view of a known sensing arrangement,illustrating problems of defocus and telecentricity;

FIG. 6 is a schematic detail of one sensor in the known sensingarrangement of FIG. 5;

FIG. 7 is a schematic side view of the sensing arrangement in theapparatus of FIGS. 1 to 4, showing the application of light-fieldimaging;

FIG. 8 is a schematic detail of one sensor in the sensing arrangement ofFIG. 6, showing one implementation of light-field imaging;

FIG. 9 is a flowchart of part of a device manufacturing method using theapparatus of FIG. 1;

FIG. 10 is a schematic detail of a modified sensor operable in a normalmode and a light-field mode;

FIG. 11 is a schematic detail of another modified sensor operable innormal and light-field modes; and

FIG. 12 is a flowchart of a height sensing method using a light-fieldimaging sensor.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 schematically depicts a lithographic apparatus LA. The apparatusincludes an illumination system (illuminator) IL configured to conditiona radiation beam B (e.g., UV radiation or DUV radiation), a patterningdevice support or support structure (e.g., a mask table) MT constructedto support a patterning device (e.g., a mask) MA and connected to afirst positioner PM configured to accurately position the patterningdevice in accordance with certain parameters; a substrate table (e.g., awafer table) WT constructed to hold a substrate (e.g., a resist coatedwafer) W and connected to a second positioner PW configured toaccurately position the substrate in accordance with certain parameters;and a projection system (e.g., a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g., including one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The patterning device support holds the patterning device in a mannerthat depends on the orientation of the patterning device, the design ofthe lithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The patterning device support can use mechanical, vacuum, electrostaticor other clamping techniques to hold the patterning device. Thepatterning device support may be a frame or a table, for example, whichmay be fixed or movable as required. The patterning device support mayensure that the patterning device is at a desired position, for examplewith respect to the projection system. Any use of the terms “reticle” or“mask” herein may be considered synonymous with the more general term“patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. Generally, the pattern imparted to theradiation beam will correspond to a particular functional layer in adevice being created in the target portion, such as an integratedcircuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.,employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g., employing a programmable mirror array of a typeas referred to above, or employing a reflective mask).

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the patterning device support (e.g., mask tableMT), and is patterned by the patterning device. Having traversed thepatterning device (e.g., mask) MA, the radiation beam B passes throughthe projection system PS, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioner PW andposition sensor IF (e.g., an interferometric device, linear encoder, 2-Dencoder or capacitive sensor), the substrate table WT can be movedaccurately, e.g., so as to position different target portions C in thepath of the radiation beam B. Similarly, the first positioner PM andanother position sensor (which is not explicitly depicted in FIG. 1) canbe used to accurately position the patterning device (e.g., mask) MAwith respect to the path of the radiation beam B, e.g., after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe patterning device support (e.g., mask table) MT may be realized withthe aid of a long-stroke module (coarse positioning) and a short-strokemodule (fine positioning), which form part of the first positioner PM.Similarly, movement of the substrate table WT may be realized using along-stroke module and a short-stroke module, which form part of thesecond positioner PW. In the case of a stepper (as opposed to a scanner)the patterning device support (e.g., mask table) MT may be connected toa short-stroke actuator only, or may be fixed.

Patterning device (e.g., mask) MA and substrate W may be aligned usingmask alignment marks M1, M2 and substrate alignment marks P1, P2.Although the substrate alignment marks as illustrated occupy dedicatedtarget portions, they may be located in spaces between target portions(these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice (e.g., mask) MA, the mask alignment marks may be located betweenthe dies. Small alignment markers may also be included within dies, inamongst the device features, in which case it is desirable that themarkers be as small as possible and not require any different imaging orprocess conditions than adjacent features.

The depicted apparatus could be used in a variety of modes. In a scanmode, the patterning device support (e.g., mask table) MT and thesubstrate table WT are scanned synchronously while a pattern imparted tothe radiation beam is projected onto a target portion C (i.e., a singledynamic exposure). The velocity and direction of the substrate table WTrelative to the patterning device support (e.g., mask table) MT may bedetermined by the (de-)magnification and image reversal characteristicsof the projection system PS. In scan mode, the maximum size of theexposure field limits the width (in the non-scanning direction) of thetarget portion in a single dynamic exposure, whereas the length of thescanning motion determines the length (in the scanning direction) of thetarget portion. Other types of lithographic apparatus and modes ofoperation are possible, as is well-known in the art. For example, a stepmode is known. In so-called “maskless” lithography, a programmablepatterning device is held stationary but with a changing pattern, andthe substrate table WT is moved or scanned. Each target portion iscommonly referred to as a “field”, and contains one or more product diesin the finished product.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Lithographic apparatus LA in this example is of a so-called dual stagetype which has two substrate tables WTa, WTb and two stations—anexposure station EXP and a measurement station MEA—between which thesubstrate tables can be exchanged. While one substrate on one substratetable is being exposed at the exposure station, another substrate can beloaded onto the other substrate table at the measurement station andvarious preparatory steps carried out. The preparatory steps may includemapping the surface control of the substrate using a level sensor LS andmeasuring the position of alignment markers on the substrate using analignment sensor AS.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to a lithocell or cluster,which also includes apparatus to perform pre- and post-exposureprocesses on a substrate. Conventionally these include spin coaters SCto deposit resist layers, developers DE to develop exposed resist, chillplates CH and bake plates BK. A substrate handler, or robot, RO picks upsubstrates from input/output ports I/O1, I/O2, moves them between thedifferent process apparatus and delivers then to the loading bay LB ofthe lithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU which is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithography controlunit LACU. Thus, the different apparatus can be operated to maximizethroughput and processing efficiency. The substrates processed by thetrack are then transferred to other processing tools for etching andother chemical or physical treatments within the device manufacturingprocess.

The lithographic apparatus control unit LACU controls all the movementsand measurements of the various actuators and sensors described. LACUalso includes signal processing and data processing capacity toimplement desired calculations relevant to the operation of theapparatus. In the terminology of the introduction and claims, thecombination of these processing and control functions referred to simplyas the “controller”. In practice, control unit LACU will be realized asa system of many sub-units, each handling the real-time dataacquisition, processing and control of a subsystem or component withinthe apparatus. For example, one processing subsystem may be dedicated toservo control of the substrate positioner PW. Separate units may evenhandle coarse and fine actuators, or different axes. Another unit mightbe dedicated to the readout of the position sensor IF. Overall controlof the apparatus may be done by a central processing unit, communicatingwith these sub-systems processing units, with operators and with otherapparatuses involved in the lithographic manufacturing process.

The lithographic apparatus of FIG. 1 includes novel sensing arrangementto implement the alignment sensor AS. This will be described below,after some further background.

FIG. 3 illustrates the steps to expose target portions (e.g. dies) on asubstrate W in the dual stage apparatus of FIG. 1. The process accordingto conventional practice will be described first.

On the left hand side within a dotted box are steps performed at ameasurement station MEA, while the right hand side shows steps performedat the exposure station EXP. From time to time, one of the substratetables WTa, WTb will be at the exposure station, while the other is atthe measurement station, as described above. For the purposes of thisdescription, it is assumed that a substrate W has already been loadedinto the exposure station. At step 200, a new substrate W′ is loaded tothe apparatus by a mechanism not shown. These two substrates areprocessed in parallel in order to increase the throughput of thelithographic apparatus.

Referring initially to the newly-loaded substrate W′, this may be apreviously unprocessed substrate, prepared with a new photo resist forfirst time exposure in the apparatus. In general, however, thelithography process described will be merely one step in a series ofexposure and processing steps, so that substrate W′ has been throughthis apparatus and/or other lithography apparatuses, several timesalready, and may have subsequent processes to undergo as well. Animportant performance parameter of the lithographic process is overlay,which is a measure of how accurately features defined by one patterningstep are positioned in relation to features formed on the substrate byprevious pattering steps. Particularly for the problem of improvingoverlay performance, the task is to ensure that new patterns are appliedin exactly the correct position on a substrate that has already beensubjected to one or more cycles of patterning and processing. Theseprocessing steps progressively introduce distortions in the substratethat must be measured and corrected for, to achieve satisfactory overlayperformance.

The previous and/or subsequent patterning step may be performed in otherlithography apparatuses, as just mentioned, and may even be performed indifferent types of lithography apparatus. For example, some layers inthe device manufacturing process which are very demanding in parameterssuch as resolution and overlay may be performed in a more advancedlithography tool than other layers that are less demanding. Thereforesome layers may be exposed in an immersion type lithography tool, whileothers are exposed in a ‘dry’ tool. Some layers may be exposed in a toolworking at DUV wavelengths, while others are exposed using EUVwavelength radiation.

At 202, alignment measurements using the substrate marks P1 etc. andimage sensors (not shown) are used to measure and record alignment ofthe substrate relative to substrate table WTa/WTb. In addition, severalalignment marks across the substrate W′ will be measured using alignmentsensor AS. These measurements are used in one embodiment to establish a“wafer grid”, which maps very accurately the distribution of marksacross the substrate, including any distortion relative to a nominalrectangular grid. The step 202 using a novel alignment sensingarrangement AS will be described in more detail below.

At step 204, a map of wafer height (Z) against X-Y position is measuredalso using the level sensor LS. The height map is used primarily toachieve accurate focusing of the exposed pattern.

When substrate W′ was loaded, recipe data 206 were received, definingthe exposures to be performed, and also properties of the wafer and thepatterns previously made and to be made upon it. To these recipe dataare added the measurements of wafer position, wafer grid and height mapthat were made at 202, 204, so that a complete set of recipe andmeasurement data 208 can be passed to the exposure station EXP. Themeasurements of alignment data for example comprise X and Y positions ofalignment marks formed in a fixed or nominally fixed relationship to theproduct patterns that are the product of the lithographic process. Thesealignment data, taken just before exposure, are combined andinterpolated to provide parameters of an alignment model. Theseparameters and the alignment model will be used during the exposureoperation to correct positions of patterns applied in the currentlithographic step. A conventional alignment model might comprise four,five or six parameters, together defining translation, rotation andscaling of the ‘ideal’ grid, in different dimensions. As describedfurther in US 2013230797A1, advanced models are known that use moreparameters.

At 210, wafers W′ and W are swapped, so that the measured substrate W′becomes the substrate W entering the exposure station EXP. In theexample apparatus of FIG. 1, this swapping is performed by exchangingthe substrate tables WTa and WTb within the apparatus, so that thesubstrates W, W′ remain accurately clamped and positioned on thosesupports, to preserve relative alignment between the substrate tablesand substrates themselves. Accordingly, once the tables have beenswapped, determining the relative position between projection system PSand substrate table WTb (formerly WTa) is all that is necessary to makeuse of the measurement information 202, 204 for the substrate W(formerly W′) in control of the exposure steps. At step 212, reticlealignment is performed using the mask alignment marks M1, M2. In steps214, 216, 218, scanning motions and radiation pulses are applied atsuccessive target locations across the substrate W, in order to completethe exposure of a number of patterns.

By using the alignment data and height map obtained at the measuringstation in the performance of the exposure steps, these patterns areaccurately aligned with respect to the desired locations, and, inparticular, with respect to features previously laid down on the samesubstrate. The exposed substrate, now labeled W″ is unloaded from theapparatus at step 220, to undergo etching or other processes, inaccordance with the exposed pattern. Depending on the implementation,the measurement steps 202, 204 do not have to be performed in thatparticular order, and can also be performed in parallel.

Alignment Sensing Arrangement

FIG. 4 shows one example of an alignment sensing arrangement AS used inthe lithographic apparatus of FIG. 1. This arrangement incorporatesmultiple alignment heads AL1 and AL21, AL22, AL23 and AL24. Eachalignment head includes an image-based alignment sensor of a type to bedescribed in more detail below. Different numbers and arrangements ofalignment heads are possible. The alignment heads are shown genericallyin FIG. 4 as being positioned over a substrate table 300 which can befor example the substrate table WTa or WTb as shown in previous figures,or other wafer stages for example.

Substrate table 300 is shown as holding a substrate 302 (substrate W′ inthe example of FIG. 3). In this example, five alignment heads areprovided. A central alignment head AL1 forms part of a primary alignmentsensor and so is referred to as a “primary alignment head”, while theouter alignment heads AL21, AL22, AL23 and AL24 form part of a secondaryalignment sensors, and so are referred to as “secondary alignmentheads”. Also shown in FIG. 4 is a level sensor 306 (LS in FIG. 1) withradiation source 308 and radiation detector 310, which will be describedin more detail later.

The alignment heads AL1, AL21, AL22, AL23, AL24 are attached to areference frame RF (not shown in FIG. 4, for clarity). The manner inwhich they are used is substantially as described in the prior publishedapplications US2008043212A1 (Shibazaki) and US2011013165A1 (Kaneko). Thedisclosure of those prior applications is hereby incorporated herein byreference. As in the examples described in the prior patentapplications, the reference frame in the present example also cariesencoder sensors, not shown in the present FIG. 4. The encoder sensorsare all fixedly attached to the projection system PS and are used totrack the position of substrate table in X, Y and Z directions. Eachencoder sensor is positioned to detect the substrate table position witha diffraction grating provided on the substrate table. The encoders maybe used instead of or in conjunction with interferometer sensors, suchas the one shown schematically at IF in FIG. 1. In alternativeembodiments, encoder sensors may be mounted on the moving substratetable 300, while encoder plates (diffraction gratings or the like) aremounted on the projection system PS or reference frame RF.

As illustrated in FIG. 4(b), an alternative embodiment of lithographicapparatus is also disclosed. This type of apparatus does not have twoidentical substrate tables 300, but rather a substrate table 300 a and aseparate measurement table 300 b that can be docked with the substratetable. Each of the measurement table and substrate table is provide withits own positioning system, based on linear motors for effecting coarseand fine motion in X and Y directions. This type of arrangement is alsodescribed in the prior applications by Shibazaki and Kaneko, and neednot be described here in detail. Different arrangements are of coursepossible.

For the purposes of the following description, the position sensingarrangements can be considered as part of an apparatus as shown in FIGS.1, 3 and 4(a), or an apparatus as shown in FIG. 4(b).

As in the known lithographic apparatus, each alignment head AL1, AL21,AL22, AL23, AL24 comprises an image-based alignment sensor which isdesigned to detect an alignment mark, which can be provided on thesubstrate (P1, P2 in FIG. 1) or on the substrate table; or on a separatemeasurement stage if applicable. As mentioned already, an alignment markcan be printed in the scribe lanes which run between die areas (targetportions C) on the substrate. It is also possible to use as an alignmentmarker a feature of the product pattern formed on the substrate, or touse specific alignment marks which are printed within the die areasthemselves.

It should further be noted that the alignment heads can and probablywill be used to measure the positions and/or other characteristics ofmarks formed on the substrate table 300/300 a and/or a measurement table300 b, not only on production substrates. Marks may be provided oncalibration plates and the like. Accordingly the term “substrate” inthis context is not intended to be limited to the production substratessuch as a silicon wafer.

The secondary alignment heads AL21, AL22, AL23, AL24 are moveable in theX direction. These relative movements between the alignment heads allowup to five alignment marks to be measured in parallel, with differentspacings. In one embodiment each of the secondary alignment heads AL21,AL22, AL23, AL24 includes an arm 320 that can turn around a rotationcenter in a predetermined angle range in clockwise and counterclockwisedirections (rotation about center 322 is marked on alignment head AL21).The X axis position of the secondary alignment heads AL21, AL22, AL23,AL24 can also be adjusted by a drive mechanism that drives the secondaryalignment heads back and forth in the X direction. It is also possiblefor the secondary alignment heads to be driven in the Y direction. Oncethe arms of the secondary alignment sensing arrangements are moved to agiven location a fixing mechanism is selectively operable to hold thearms in position. The desired positions of the alignment heads for aparticular substrate (or batch of substrates) will be specified in therecipe data described above with reference to FIG. 3, along with otherparameters of the alignment process. While the alignment sensorillustrated here comprises five alignment heads, of course other numbersof alignment heads could be used, including both odd and even numbers.

An alignment operation using the alignment heads and encoder embodimentsis described in detail in the prior applications. Broadly speaking, thesubstrate table is positioned at different positions, and differentsubsets of the alignment heads are used to detect alignment marks on thesubstrate. A number of measurement positions can be defined along theY-axis with the multiple alignment heads measuring multiple alignmentmarks at each position. The more positions that are chosen the moreaccurate the system can be, although the more time consuming thealignment process will be. For example, it is possible to define sixteenalignment marks in successive rows along the X-axis on the substratecomprising three, five, five and three marks respectively which can thenbe detected by four different alignment positions which make use ofthree, five, five and three alignment heads respectively. The number ofrows of alignment marks can be fewer or more than five, and can even beas high as many hundreds.

The data from the alignment sensors AL1, AL21, AL22, AL23, AL24 can thenbe used by a computer to compute an array of all the alignment marks onthe substrate in a co-ordinate system that is set by the measurementaxis of the x and y encoders and the height measurements by performingstatistical computations in a known manner using the detection resultsof the alignment marks and the corresponding measurement values of theencoders, together with a baseline calibration of the primary alignmentsensing arrangement and secondary alignment sensing arrangement, whichare discussed in more detail in the prior applications. The result ofthis computation is then used in an alignment model to controlpatterning steps at the exposure station, as described already withreference to FIG. 3.

Conventional Image-Based Alignment Sensor

Referring now to the side view of FIG. 5, we see the five alignmentheads AL1, AL21, AL22, AL23, AL24 of the known alignment sensingarrangement 320, mounted on reference frame RF. Although they arenominally all positioned perfectly in X and Y directions, and alignedwith their optical axes O parallel to the Z axis, in reality, slightmisalignments will exist. For example, sensor AL22 is shown with itsoptical axis tilted at an angle θ relative to the Z axis (θ may ofcourse represent a tilt about the X and/or Y axes, though only the Yaxis tilt is visible in this view). Particularly in an example where thealignment heads are movable, these slight misplacements and tilts may bedifferent from batch to batch. The measurements carried out by multiplealignment heads are carried out simultaneously where possible. However,due to the differing height along the surface of a substrate, a levelingprocess is typically carried out. This can be performed by moving thesubstrate table in the Z direction using the positioning system PW inFIG. 1. Alternatives to this will be discussed below. Level sensor LS(elements 306, 308, 310) is provided which uses a focus detectiontechnique to determine when the substrate is in line with apredetermined focal plane of the leveling sensor.

The surface of a substrate 302 is not a flat plane and has someunevenness for example due to manufacturing tolerances, clampingdistortion and unevenness introduced by the product features and theprocesses used to form them. This means in the known apparatus that atleast one alignment head generally performs detection of an alignmentmark out of focus. FIG. 5 shows an exaggerated example of this, wherethe middle alignment head AL1 is accurately focused on the substratesurface, but the other heads are out of focus with respect to the unevensubstrate 302 surface. There may also be variations in focus height ofthe individual sensors, even if the substrate were perfectly flat andlevel.

FIG. 6 shows in in schematic form the functional elements of onealignment sensor in the known alignment sensor. The sensing arrangementilluminates a subject alignment mark using a broadband detection beamthat does not expose the resist layer on a substrate, and picks up animage of the alignment mark using an imaging optical system 400 and animage detector 402. While optical system 400 is represented in thedrawing by a simple lens element, the skilled reader will understandthat the optical system will be more elaborate in a real implementation,and may include refractive and/or reflective elements. The opticalsystem is shown with the Z axis horizontal, for convenience. The drawingis made in the Z-X plane, but would look similar if drawn in the Z-Yplane. In a practical implementation, there may in fact be foldingmirrors so that the optical axis O of the sensor deviates from avertical to a horizontal path, for example to reduce height of theoverall arrangement. Such details are within the ordinary capabilitiesof the optical system designer and will not be discussed further here.

Image detector 402 comprises a two-dimensional array of photosensitiveelements 404 that may conveniently be referred to as pixels. A firstobject point of lies in a plane with height z1 which happens to coincidewith a focal plane of the alignment sensor. Optical system 400 gathersrays r1 from point of and focuses them to an image point i1 which isperfectly in the plane of the pixels 404. If the alignment mark whoseposition is being measured lies in the plane z1, then a perfectlyfocused image of the mark is formed on the array of pixels 404. Aprocessing unit 406 receives image data from all the pixels of detector402 and processes it to recognize the mark and compute its position inthe X-Y plane. The results of this computation are shown schematicallybeing delivered as data Ax and Ay. Processing unit 406 may be separatefor each sensor, or it may be a central processor of the alignmentsensing arrangement, or it may be a function implemented by thelithographic apparatus control unit LACU mentioned above.

As described already with reference to FIG. 5, an alignment mark may notalways lie in the focal plane of the sensor. In FIG. 6, a second objectpoint o2 is shown lying in a plane z2 which is (for example) furtherfrom the sensor than z1. Rays r2 from this point are not focused in theplane of pixels 404, but somewhat in front of it. Consequently, when analignment mark lies out of the focal plane z1, the image detected bydetector 402 is blurred and processing unit 406 cannot so accuratelyrecognize the mark, or determine its position accurately.

To address this problem in the known apparatus, alignment marks are readwith the substrate in several different Z positions, as illustrated at302′ and 302″ in FIG. 5. Changing the relative position in the Z-axis ofthe substrate table allows each of the alignment heads to make ameasurement in a focused state, and the prior patent applicationsdescribe the procedures and computations that are made to obtain thebest position measurement for each mark from among the severalmeasurements made. Additionally, displacement from the ideal focal planecauses the apparent X-Y position to shift. This may be due tonon-telecentricity in optical system 400, and/or to the tilt withrespect to the Z-axis of the optical axis of each of the alignmentheads. Referring to the mark 340 shown in FIG. 5 as an example, thenon-zero tilt angle θ means that the apparent X-Y position of the markchanges as the substrate is moved from position 302, to 302′ and 302″,as well as it becoming out of focus. Calibration measurements can bemade for tilt effects, so that the detection results of positions of thealignment marks can be corrected based on the measurement results.

While the known procedures and corrections provide accurate alignmentresults, each movement in the Z-axis which is performed results in anadditional step and increases the overall time for the alignment.Moreover, since different measurements of the same make are taken atdifferent times, accuracy can be degraded by drift due to mechanicaland/or thermal effects.

Alignment Sensing Arrangement with Light-Field Imaging

FIG. 7 illustrates a modified alignment sensing arrangement 530 thatforms the alignment sensor AS in the apparatus of FIGS. 1 to 4. Thearrangement comprises five alignment heads AL1, AL21, AL22, AL23, AL24,as in the known example. However, the alignment sensor within eachalignment head differs from the known sensors in its optical system anddata processing. Simply stated, each alignment sensor in the novelarrangement comprises a light-field imaging system (also known as aplenoptic imaging system) rather than a conventional 2-D imaging system.As illustrated schematically in FIG. 5, each light-field image sensorcan image marks within an extended zone 532, rather than a single focalplane. The method described in this invention disclosure has the aim toreduce the number of images of each mark that need to be taken duringthe alignment step. Potentially, a single exposure will be sufficient toobtain focused images of all marks. So-called 4-D light-fieldinformation is captured by detector 602, and focusing and otheradjustments can be performed digitally within processing unit 606. Thismeans alignment duration and drift can be reduced. Alternatively or inaddition, the light-field images can be processed to correct for tiltand telecentricity. Further they can be used to perform or assist heightmeasurement (level sensor).

FIG. 8 shows the modified optical system 600 and processing unit 606 ofone sensor in the arrangement of FIGS. 4 and 7, in a first example. Aprimary optical system 610 is provided which for ease of understandingcan be considered identical to the “normal” optical system 400 in theknown sensor. In the modified optical system 600, the image detector 602with pixels 604 is not placed in a focal plane of the primary opticalsystem. An array of microlenses 612 is placed in a nominal focal planezf of optical system 610 (where the image detector was in theconventional sensor). Each microlens 612 corresponds in area to asub-array of the pixels 604. The microlenses are arrayed in twodimensions and may be far smaller and more numerous than the fewillustrated in the drawings. Similarly, the pixels of image detector arefar more numerous than illustrated. The array of microlens is an exampleof a light field modulating element. Other forms of light fieldmodulating elements are known for light-field imaging in other fields ofapplication, as described below.

As illustrated by rays 614, in this example, the power of each microlens612 is such as to focus a point in a pupil plane zL of optical system610 onto a point in the plane zp of the pixels 604. Consequently, whilethe image detected by detector 602 may be similar to the conventional 2Dimage on a large scale, the intensity when one looks at individualpixels depends not only on the relative brightness of object points suchas points o1 and o2, but also on the angle of rays r1 and r2 falling ateach point on the microlens array. By combining image data from pixelsselected for their positions in relation to the microlenses, not onlythe image i1 but also the image i2 can be focused digitally and objectso1 and o2 can both resolved in a digital image. Consequently, when eachof the alignment heads AL1, AL21, AL22, AL23, AL24 in FIG. 7 has asensor of the type illustrated, then five alignment marks can be focuseddigitally using information of a single exposure. Even if the range ofthis “digital focusing” is not sufficient for the large heightvariations across a particular substrate, the number of images requiredat different Z-positions would be reduced.

This light-field imaging technique (based on plenoptic imaging) has beenapplied in microscopy to allow “3D-studies” of biological specimens. Inalignment sensors for photolithography, the use of light-field imaginghas not been reported. Only a 2-D image is captured at the sensor plane.More detail of light-field microscopy can be found in the paper Levoy etal, “Light Field Microscopy”, ACM Transactions on Graphics 25(3), Proc.SIGGRAPH 2006 and on the Stanford University website athttp://graphics.stanford.edu/papers/lfmicroscope/. A published patentapplication WO 2007/044725 describes substantially the same work. Asexplained by Levoy et al, each micro-lens measures not just the totalamount of light deposited at that location, but how much light arrivesalong each ray (angular direction). By re-sorting the measured rays oflight to where they would have terminated in traditional camera systems,sharp images can be computed with different depths-of-focus. Thus, thefinal field image of the alignment mark becomes a computationalcomposition of weighted pixel-subsets. Different focal depths can beselected by means of choosing different spatial pixel configurations(from single or multiple pixel subsets) to focus on the alignment markwith the optimal height. Thus, a specific pixel subset corresponds to acertain focus height. The subset configuration can be stored in alook-up table after calibration. This property allows extending thedepth-of-field of the image sensor without reducing the aperture andenabling single exposure field image alignment, without adjusting theheight of the substrate table itself.

Additionally, Levoy et al describe how selecting further differentsubsets of pixels allows a shift of viewpoint to be achieved in the X-Yplane, as well as a shift of focus. This can be used to correct tilt andtelecentricity effects, so that the position of an alignment mark orother target features in the computed image is independent of depth.

FIG. 9 is a simplified flowchart summarizing part of a devicemanufacturing method exploiting the novel alignment sensing arrangementas described above in one exemplary embodiment. This diagram can be readin conjunction with FIG. 3. Substrates are loaded in to the lithographicapparatus measured using alignment sensor arrangement AS and levelsensor LS. In the dual-stage example of FIG. 1, step PAT is performed atmeasurement station MEA. Using the 4-D light-field image data collectedby the sensors in each alignment head, alignment positions Ax and Ay fornumerous marks are calculated in a digital focusing step DFOC.

A benefit of light-field imaging is that an alignment mark can bebrought into focus by each alignment head, even though physically theyare at different distances from the sensor. Another benefit is thatdepth information Az can be obtained which allows processing unit 606 tocorrect displacements in the apparent X-Y position of a mark basedknowledge of the telecentricity and/or tilt angle of the individualalignment heads. Tilt and telecentricity can be measured in acalibration sequence, in which substrate 302 is measured at differentheights, as shown in FIG. 5.

An alignment model ALM based on the alignment data is used to controlthe positioning of an applied pattern in the patterning step PAT. In thedual-stage example of FIG. 1, step PAT is performed at exposure stationEXP. The height map data from the level sensor LS is used in a focuscontrol model FOC to control focus in the patterning step. After thepattern is applied to each target portion in patterning step PAT, thesubstrate is processed to create device features in accordance with thepattern. The substrate returns for further patterning until all productlayers are complete. Additionally, an advanced process control moduleAPC can use metrology data from an inspection apparatus MET such as ascatterometer, to update control of the process.

The method described above, enables field image alignment by means of asingle exposure. Reducing the number of image required during thealignment process is beneficial for throughput and overlay. Throughputis improved simply because less measurement time per mark is required.Overlay performance is improved because alignment of each layer is lessaffected by drift between multiple images required for alignmentmeasurements in the known arrangements.

In addition to correcting focus and telecentricity, other aberrations ofoptical system 610 can be corrected by digital processing. Sphericalaberrations for example are digitally correctable in a similar way asrefocusing.

Further, multiple focus planes can be selected simultaneously, withinthe same alignment sensor. Therefore underlying structures can be set tofocus at the same time. This can be used for example to measure overlaybetween marks formed in two layers on the substrate.

The example light-field imaging sensors illustrated in FIG. 8 are basedon microlenses as the simplest implementation of the light-field imagingconcept. However, other implementations of light-field imaging areknown, for example those based on coded masks. Coded masks may beregarded as an example of spatial light modulators (SLMs). Knownexamples comprise arrays of square elements which are either opaque ortransparent, according to a predefined pattern. Knowledge of the patterncan be used to reconstruct different images from a recorded light field,in the same way as knowledge of the microlens array can be used in theexample of FIG. 8. As is known, programmable SLMs can be implemented bytransmissive elements (for example, liquid crystal SLM) and/orreflective (for example, deformable micromirror device). In generalterms, the microlens array, coded masks, SLMs can be regarded asexamples of light field modulating elements. Different implementationsbring different combinations of imaging performance parameters such asspatial resolution, noise sensitivity, light usage and so forth. Inparticular, while a simple microlens implementation generally leads to aloss of spatial resolution that may be undesirable in the alignmentsensor application, mask-based imaging offers full resolution. Codedmask may be placed at various locations in the optical path, for examplein place of the illustrated microlens array, in superposition with amicrolens array, and/or in the pupil plane of the primary optical system610. Coded masks and/or microlens arrays may be placed in anillumination light path, as well as an imaging light path.

The following publications provide theoretical and experimentalteachings in relation to light-field imaging, primarily in the field ofphotography.

K. Marwah, G. Wetzstein, Y. Bando, R. Raskar, “Compressive Light FieldPhotography using Overcomplete Dictionaries and Optimized Projections”,Proc. of SIGGRAPH 2013 (ACM Transactions on Graphics 32, 4), 2013.

A. Ashok, Mark A. Neifeld, “Compressive light-field imaging”, SPIENewsroom, DOI: 10.11117/2.1201008.003113, 19 Aug. 2010.

O. Cossairt, M. Gupta, Shree K Nayar, “When Does Computational ImagingImprove Performance?”, IEEE Transactions on Image Processing (Volume 22,Issue 2), February 2013, Pages 447-458, DOI: 10.1109/TIP.2012.2216538.

I. Ihrke, G. Wetzstein, W. Heidrich, “A theory of plenopticmultiplexing”, 2010 IEEE Computer Society Conference on Computer Visionand Pattern Recognition, pp 483-490, 13-18 Jun. 2010, San Francisco,Calif., http://doi.ieeecomputersociety.org/10.1109/CVPR.2010.5540174.

T. E. Bishop, S. Zanetti, P. Favaro, “Light Field Superresolution”, in1st IEEE International Conference on Computational Photography (ICCP),April 2009, Pages 1-9, DOI: 10.1109/ICCPHOT.2009.5559010.

Z. Xu, J. Ke, E. Y Lam, “High-resolution lightfield photography usingtwo masks”, Optics Express. May 2012 7; 20(10):10971-83. DOI:10.1364/OE.20.010971.

M. W. Tao, S. Hadap, J. Malik, and R. Ramamoorthi, “Depth from CombiningDefocus and Correspondence Using Light-Field Cameras”. Proceedings ofInternational Conference on Computer Vision (ICCV), 2013.

The skilled reader can adapt these teachings to the sensing and controlarrangements for alignment and/or focusing in a lithographic apparatus,or similar measurement and control functions in other apparatuses. Inparticular, these teachings enable the skilled reader to select acombination of optical system and processing techniques that provide adesired trade-off between light levels, spatial resolution and the like.

FIG. 10 shows a modified version of the light-field image sensor opticalsystem 600 of FIG. 8, adapted to allow either light-field imaging fordigital focusing or conventional imaging to be performed by thealignment sensing arrangement. Elements are the same as in FIG. 8,except that the array of microlenses 612 is moveable out of the opticalpath. A path length correction element 620 is swapped into the opticalpath, so as to focus the primary optical system 610 onto the pixels 604instead of the (absent) microlens array. Alternatively, image detector602 could be moved forward or backward to achieve the same effect. Asalready mentioned, light-field imaging can involve a compromise betweendifferent performance parameters. For example, the ability to refocusdigitally from a single exposure might be gained at the expense ofreduce performance in light capture, or spatial resolution.

Being able to select light-field imaging or conventional 2-D imagingallows the operator to choose whether to exploit the benefits of thenovel light-field image sensors or to exploit the benefits of theconventional imaging system. Another option is to use both types ofimaging on the same substrate, for example to calibrate the light-fieldimaging function against the conventional imaging function. Selection ofthe mode to use and calibration can be automated. In addition toselecting between light-field and non-light-field imaging modes, thesensors can be switched between different modes of light-field imaging,for example using different techniques from among those described in thereferences cited above.

FIG. 11 illustrates another modification of the light-field image sensoroptical system 600 in which separate branches are provided in theoptical system for light-field imaging and conventional 2-D imaging. Abeam diverting element 630 diverts rays away from the light-fieldimaging path to a second optical system 632 and second image detector634. A second processing unit 606′ is indicated, which processes imagedata from the detector 634, although units 606 and 606′ can of course betwo functions of a single physical processing unit. Beam divertingelement may be for example a movable mirror or prism, by which anoperator can select whether to use light-field imaging or conventionalimaging. Beam diverting element may on the other hand be a beam splittersuch that light from the alignment mark is processed by both thelight-field imaging branch and the conventional imaging branch at thesame time. Results of processing unit 606 may for example tell which ofthe marks imaged by conventional imaging branch are in focus.

Height Sensing by Light-Field Imaging

As shown in a broken line in FIG. 8, the digital processing at step DFOCcan also produce height data Az to supplement or potentially replacethat provided by level sensors LS. That is to say, by recognizing thedepth of best focus for each image of each alignment mark, or for otherfeatures on the substrate, a height value for that mark can be derived,in addition to an X-Y position. In principle, the light-field imagesensors could be used entirely for height sensing to control focus inthe patterning step, but their primary function in the present exampleis alignment to control X-Y position in the patterning step.

FIG. 12 is a more detailed flowchart showing the extraction of depthinformation for one mark in one light-field imaging sensor of the typeshown in FIGS. 7 and 8. The method is based on that of the Tao et alpaper, referenced above. A light-field image is captured in a step CAP,using the optical system 610 and image detector 602. Two types of depthanalysis are then performed on the light-field image data, and theirresults combined. A defocus algorithm DEFALG calculates defocus cuesDEFCUE and a correspondence algorithm CORALG calculates correspondencecues CORCUE. These different types of cues are then combined to obtain adepth value, for example the value Az referred to in earlierdescription. As described by Tau, an algorithm that exploits both typesof analysis is more robust than either analysis alone. This method isrepeated to calculate depth values for every position in the image. Asindicated by the broken path, flow returns to capture new images untilall positions have been measured. The flow may return to capture a newimage at the same position if the quality of captured data is deemed notsufficient. The flow may return to capture a new image at the sameposition if the depth of focus in the light-field image data is notgreat enough to cover all possible depths of focus in a singlelight-field image.

The proposed use of light-field imaging is not limited to the 2Dalignment purpose, but can also be exploited in a more general 3Dmetrology application. That is, depth or height information can beextracted from the light-field image, since both defocus andcorrespondence depth cues are available simultaneously in one capture.Height maps in lithography are typically based on fringe projection andinterferometry, which are recorded separately from, or in parallel with,alignment measurements. As is mentioned above, refocused images can beconstructed after acquisition of images captured using a lens let arrayor other light field modulating element(s), as well as obtainingmultiple views by shifting digitally viewpoint within the aperture ofthe main objective lens, for example to compensate telecentricity.Combining both defocus and correspondence cues of a single exposure,height maps of desired features can be computed.

The skilled person can readily adapt the depth camera techniques of Tauet al to perform a height mapping function in the lithographic apparatusof FIGS. 1 to 4. The light-field image sensor(s) used for this may bededicated depth sensor(s), or it/they may be the same as the alignmentsensors used for X-Y position measurement. The depth sensors may be usedinstead of or as a supplement to the optical level sensor LSconventionally used in lithography, as already mentioned.

CONCLUSIONS

In conclusion, the novel method, for obtaining measurement s to controla patterning process in lithography can reduce measurement time and sohelp maintain a high throughput of products.

An embodiment of the invention may be implemented using a computerprogram containing one or more sequences of machine-readableinstructions describing a methods of controlling the lithographicapparatus using alignment and/or height map data obtained by light-fieldimaging as described above. This computer program may be executed forexample within the control unit LACU of FIG. 2, or some othercontroller. There may also be provided a data storage medium (e.g.,semiconductor memory, magnetic or optical disk) having such a computerprogram stored therein. Where an existing lithographic apparatus, forexample of the type shown in FIG. 1, is already in production and/or inuse, the invention can be implemented by the provision of updatedcomputer program products for causing a processor to perform themodified image processing step DFOC of a method shown in FIG. 9 and/orthe steps of FIG. 12.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography,topography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

Digital refocusing for field image alignment is explained above withreference to lithographic apparatus, i.e., an apparatus for exposing asubstrate to a patterned beam of radiation, the substrate being coveredwith a photo-sensitive resist. For completeness, it is remarked herethat such digital refocusing can also be applied in metrology apparatusfor, e.g., diffraction based overlay (DBO) and diffraction based focus(DBF) analysis. A metrology apparatus subjects an exposed substrate tooperations for analyzing the quality of the result of the exposure interms of, e.g., overlay and/or focus. A system with two or more camerascan be applied with each individual detection branch (completelyseparated or with a main optical path as common detection branch) foroptimal performance for feature to be detected. As a result, informationabout polarization, pupil plane and field can be extracted. The benefitsof the invention applied to the metrology tool are similar to thebenefits of application of the invention in a lithographic apparatus:image focus correction is done by software. The image can then berefocused without moving objective or moving other parts, resultingeventually in an increase in throughput. Focus errors, introduced byhardware, can also be reduced.

In an embodiment, there is provided a lithographic apparatus forapplying a pattern onto a substrate, the apparatus including: at leastone sensor for measuring positions of features on the substrate prior toapplying the pattern, the sensor comprising an imaging optical system,and an image detector for capturing an image formed by the imagingoptical system; a data processor for extracting position measurementsfrom pixel data supplied by the image detector; and a controllerarranged to control the lithographic apparatus to apply the pattern tothe substrate using the positions measured by the sensor, wherein theimaging optical system includes one or more light field modulatingelements and the data processor is arranged to process the pixel data asa light-field image to extract the position measurements.

In an embodiment, the data processor is arranged to derive from thelight-field image a focused image of a feature on the substrate, and toindicate a position of the feature based on the focused image indirections transverse to an optical axis of the imaging optical system.In an embodiment, the data processor is arranged to include in theposition measurement a correction to reduce a depth dependency of anapparent position of the feature in the detected image. In anembodiment, the data processor is arranged to derive from thelight-field image an image of a feature on the substrate with aviewpoint corrected. In an embodiment, the data processor is arranged toderive from the light-field image a measurement of height of a featureon the substrate, the dimension of height being substantially parallelto an optical axis of the imaging optical system, the controller usingthe measurement of height to control focusing of a pattern applied bythe lithographic apparatus. In an embodiment, the data processor isarranged to derive from the light-field image an image focused at two ormore depths. In an embodiment, the sensor is one of a plurality ofsimilar sensors mounted on a common reference frame, the plurality ofsensors in operation measuring positions of a plurality of featuressubstantially simultaneously at respective locations across thesubstrate, the processor of each sensor deriving from its respectivelight-field image an image focused on a local portion of the substrate.In an embodiment, the sensor is one of a plurality of similar sensorsmounted on a common reference frame, the plurality of sensors inoperation measuring positions of a plurality of features substantiallysimultaneously at respective locations across the substrate, theprocessor of each sensor deriving from its respective light-field imagean image adjusted for a sensor-specific dependency between position anddepth.

In an embodiment, there is provided a device manufacturing methodcomprising applying patterns in successive layers on a substrate, andprocessing the substrate to produce functional device features, whereinthe step of applying a pattern in at least one of the layers comprises:(a) measuring positions of features on the substrate in a lithographicapparatus, using at least one light-field imaging sensor; (b) extractingposition measurements from light-field image data obtained using thesensor; and (c) controlling the lithographic apparatus to apply thepattern to the substrate using the positions measured by the alignmentsensor.

In an embodiment, step (b) comprises deriving from the light-field imagedata a focused image of a feature on the substrate and indicates aposition of the feature based on the focused image in directionstransverse to an optical axis of the imaging optical system. In anembodiment, the position measurement includes a correction to reduce adepth dependency of an apparent position of the feature in the detectedimage. In an embodiment, step (b) comprises deriving from thelight-field image an image of a feature on the substrate with aviewpoint corrected. In an embodiment, step (b) comprises deriving fromthe light-field image a measurement of height of a feature on thesubstrate, the dimension of height being substantially parallel to anoptical axis of the imaging optical system, and step (c) comprises usingthe measurement of height to control focusing of a pattern applied bythe lithographic apparatus. In an embodiment, step (b) comprisesderiving from the light-field image an image focused at two or moredepths. In an embodiment, the sensor is one of a plurality of similarsensors mounted on a common reference frame, the plurality of sensorsmeasuring positions of a plurality of features substantiallysimultaneously at respective locations across the substrate, and step(b) comprises deriving from its respective light-field image an imagefocused on a local portion of the substrate. In an embodiment, thesensor is one of a plurality of similar sensors mounted on a commonreference frame, the plurality of sensors measuring positions of aplurality of features substantially simultaneously at respectivelocations across the substrate, and step (b) comprises for each sensorderiving from its respective light-field image an image adjusted for asensor-specific dependency between position and depth.

In an embodiment, there is provided a computer program productcomprising machine-readable instructions for causing one or moreprocessors to implement the data processor and controller functions of alithographic apparatus as described herein.

In an embodiment, there is provided a data processing system comprisingone or more processors programmed to implement the controller of alithographic apparatus as described herein.

In an embodiment, there is provided a computer program productcomprising machine-readable instructions for causing one or moreprocessors to perform the step (b) of the method as described above.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description by example, and not oflimitation, such that the terminology or phraseology of the presentspecification is to be interpreted by the skilled artisan in light ofthe teachings and guidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

The invention claimed is:
 1. A lithographic apparatus for applying apattern onto a substrate, the lithographic apparatus including: at leastone sensor configured to measure positions of features on the substrateprior to applying the pattern, the at least one sensor comprising animaging optical system, and an image detector for capturing an imageformed by the imaging optical system; a data processor configured toextract position measurements from pixel data supplied by the imagedetector; and a controller arranged to control the lithographicapparatus to apply the pattern to the substrate using the positionsmeasured by the at least one sensor, wherein the imaging optical systemincludes one or more light field modulating elements and the dataprocessor is arranged to process the pixel data as a light-field imageto extract the position measurements.
 2. The apparatus as claimed inclaim 1, wherein the data processor is arranged to derive from thelight-field image a focused image of a feature on the substrate, and toindicate a position of the feature based on the focused image in adirection transverse to an optical axis of the imaging optical system.3. The apparatus as claimed in claim 2, wherein the data processor isarranged to include in the position measurement a correction to reduce adepth dependency of an apparent position of the feature in the detectedimage.
 4. The apparatus as claimed in claim 1, wherein the dataprocessor is arranged to derive from the light-field image an image of afeature on the substrate with a viewpoint corrected.
 5. The apparatus asclaimed in claim 1, wherein the data processor is arranged to derivefrom the light-field image a measurement of height of a feature on thesubstrate, the dimension of height being substantially parallel to anoptical axis of the imaging optical system, the controller using themeasurement of height to control focusing of a pattern applied by thelithographic apparatus.
 6. The apparatus as claimed in claim 1, whereinthe data processor is arranged to derive from the light-field image animage focused at two or more depths.
 7. The apparatus as claimed inclaim 1, wherein the at least one sensor is one of a plurality ofsimilar sensors mounted on a common reference frame, the plurality ofsensors in operation measuring positions of a plurality of featuressubstantially simultaneously at respective locations across thesubstrate, the processor of each sensor deriving from its respectivelight-field image an image focused on a local portion of the substrate.8. The apparatus as claimed in claim 1, wherein the at least one sensoris one of a plurality of similar sensors mounted on a common referenceframe, the plurality of sensors in operation measuring positions of aplurality of features substantially simultaneously at respectivelocations across the substrate, the processor of each sensor derivingfrom its respective light-field image an image adjusted for asensor-specific dependency between position and depth.
 9. A devicemanufacturing method comprising applying patterns in successive layerson a substrate, and processing the substrate to produce functionaldevice features, wherein applying at least one of the patterns in atleast one of the layers comprises: measuring positions of features onthe substrate in a lithographic apparatus, using at least onelight-field imaging sensor; extracting position measurements fromlight-field image data obtained using the at least one light-fieldimaging sensor; and controlling the lithographic apparatus to apply theat least one pattern to the substrate using the position measurements.10. The method as claimed in claim 9, wherein the extracting comprisesderiving from the light-field image data a focused image of a feature onthe substrate and indicating a position of the feature based on thefocused image in a direction transverse to an optical axis of theimaging optical system.
 11. The method as claimed in claim 10, whereinthe position measurement includes a correction to reduce a depthdependency of an apparent position of the feature in the detected image.12. The method as claimed in claim 9, wherein the extracting comprisesderiving from the light-field image data an image of a feature on thesubstrate with a viewpoint corrected.
 13. The method as claimed in claim9, wherein the extracting comprises deriving from the light-field imagedata a measurement of height of a feature on the substrate, thedimension of height being substantially parallel to an optical axis ofthe imaging optical system, and the controlling comprises using themeasurement of height to control focusing of a pattern applied by thelithographic apparatus.
 14. The method as claimed in claim 9, whereinthe extracting comprises deriving from the light-field image data animage focused at two or more depths.
 15. The method as claimed in claim9, wherein the at least one light-field imaging sensor is one of aplurality of similar sensors mounted on a common reference frame, theplurality of sensors measuring positions of a plurality of featuressubstantially simultaneously at respective locations across thesubstrate, and the extracting comprises deriving from its respectivelight-field image data an image focused on a local portion of thesubstrate.
 16. The method as claimed in claim 9, wherein the at leastone light-field imaging sensor is one of a plurality of similar sensorsmounted on a common reference frame, the plurality of sensors measuringpositions of a plurality of features substantially simultaneously atrespective locations across the substrate, and the extracting comprisesfor each sensor deriving from its respective light-field image data animage adjusted for a sensor-specific dependency between position anddepth.
 17. A non-transitory computer program product comprisingmachine-readable instructions configured to cause a processor system toat least: process pixel data as a light-field image to extract positionmeasurements of features on a substrate in a lithographic apparatus, thepixel data obtained by an image detector capturing an image of thesubstrate formed by an imaging optical system including one or morelight field modulating elements; and control the lithographic apparatusto apply a pattern to the substrate using the position measurements. 18.A data processing system comprising one or more processors programmed toimplement a controller of a lithographic apparatus, the one or moreprocessors configured to at least: process pixel data as a light-fieldimage to extract position measurements of features on a substrate in thelithographic apparatus, the pixel data obtained by an image detectorcapturing an image of the substrate formed by an imaging optical systemincluding one or more light field modulating elements; and control thelithographic apparatus to apply a pattern to the substrate using theposition measurements.
 19. A non-transitory computer program productcomprising machine-readable instructions configured to cause one or moreprocessors to extract position measurements from light-field image dataobtained using at least one light-field imaging sensor measuringfeatures on a substrate in a lithographic apparatus.
 20. The computerprogram product of claim 19, wherein the instructions are furtherconfigured to control the lithographic apparatus to apply a pattern tothe substrate using the position measurements.